Synthesis of L‐Pyranosides from 5‐Enopyranosides by Diastereoselective Hydroboration/Oxidation.

Abstract

Synthesis of L-Pyranosides from 5-Enopyranosides by Diastereoselective Hydroboration/Oxidation. -The reaction provides a simple and efficient approach to L-idoside and L-glucoside as well as to L-iduronic acid (IV). Interestingly, in the case of 5-enoglucosides the diastereoselective outcome is strongly dependent on the amount of BH3. -(TAKAHASHI, H.; MIYAMA, N.; MITSUZUKA, H.; IKEGAMI*, S.; Synthesis 2004, 18, 2991-2994; Fac. Pharm. Sci., Teikyo Univ., Sagamiko, Kanagawa 199-01, Japan; Eng.) -Jannicke 21-204

Synthesis of Four Orthogonally Protected Rare l-Hexose Thioglycosides from d-Mannose by C-5 and C-4 Epimerization

“…The C-5 epimerization of 26 was performed by hydroboration with BH 3 ·THF complex followed by oxidation with H 2 O 2 under previously optimized conditions, using 10 equiv. of the borane reagent [ 25 , 26 , 65 , 66 , 67 ]. Although the yield of the expected l - gulo -configured product 28 was only 53%, a sufficiently high l -guloside: d -mannoside ratio was observed ( 28 : 24 ~5:1), so it was considered worthwhile to investigate the C-5 epimerization of 1-thiomannoside alkenes having other protecting group patterns.…”

“…Therefore, it is important to produce l -sugars from common carbohydrates by cost-effective and high-yielding synthetic routes. To meet the demand of l -sugars, various strategies have been developed for their synthesis [ 20 , 21 , 22 , 23 , 24 ], including C-5 epimerization of readily available d -sugars [ 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ], homologation of carbohydrates with shorter chains [ 38 , 39 , 40 , 41 , 42 ], de novo synthetic routes [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ], head to tail inversion [ 51 , 52 , 53 , 54 , 55 , 56 ], site selective epimerization [ 57 , 58 ], C-H activation [ 59 , 60 ], and enzymatic synthesis [ 61 , 62 ]. However, there are very few approaches that provide the l -sugars as glycosyl donors, e.g., in form of thioglycosides, ready for glycosylation [ 60 , 63 , 64 , 65 , …”

“…The synthesis of l -sugars by hydroboration/oxidation of d -sugar-derived 5-enopyranosides has been extensively studied on α- O -glycosides, focusing mainly on d -glucosides, and to a lesser extent also on sugars of other configurations. ( Scheme 1 A) [ 25 , 26 , 67 ]. Recently, we have successfully extended the scope of this method to thioglycosides derived from d -glucose, thus giving direct, rapid access to l -idose glycosyl donors.…”

“…While the hydroboration of d -glucoside-derived enopyranosides has been studied extensively [ 25 , 26 , 75 , 76 , 77 ], only two publications have so far addressed the hydroboration of the corresponding mannosides [ 25 , 67 ]. The BH 3 ·THF-mediated C-5 epimerization of 5-enomannoside 8 was reported to proceed with moderate diastereoselectivity [ 25 ], probably due to the steric hindrance by the C-2 substituent which hampers attack of the reagent from the β-face ( Scheme 1 A).…”

“…While the hydroboration of d -glucoside-derived enopyranosides has been studied extensively [ 25 , 26 , 75 , 76 , 77 ], only two publications have so far addressed the hydroboration of the corresponding mannosides [ 25 , 67 ]. The BH 3 ·THF-mediated C-5 epimerization of 5-enomannoside 8 was reported to proceed with moderate diastereoselectivity [ 25 ], probably due to the steric hindrance by the C-2 substituent which hampers attack of the reagent from the β-face ( Scheme 1 A). A much higher l - gulo : d - manno ratio could be achieved using catecholborane and RhCl(PPh 3 ) 3 catalysis (Wilkinson’s catalyst), but the protecting group pattern of mannoside alkenes also proved to be an important factor in l - gulo selectivity ( 11 → 12 ) [ 67 ].…”